Fibers that are used for optical communication are waveguides made of transparent dielectrics whose function is to guide visible and infrared light over long distances. An optical fiber consists of an inner cylinder made of glass, called the core, surrounded by a cylindrical shell of glass or plastic of lower refractive index, called the cladding. The cladding may be made from several layers called the barrier tube and jackets. When the cladding is secured to the core rod and the cladding is a specified diameter, the combination is called a fiber optic preform.
There are three (3) significant fabrication processes in use today to manufacture fiber optic preforms which in-turn are used to manufacture optical fibers. One process is commonly known as the lateral soot deposition technique as described in U.S. Pat. Nos. 3,711,262 and 3,876,560. In the lateral soot deposition technique, glass particulate matter and dopant halides are formed in a hydrolysis burner and deposited on a starting member such as a glass rod. Additional layers of glass including a cladding layer are deposited on the rod and the combination is consolidated onto a transparent rod by heating in an inert environment. Subsequent to the consolidation, the starting member may be withdrawn, leaving a hollow cylinder of glass which may be drawn into a fiber. This process, developed by Corning Glass Works, requires many passes (up to 200) by the hot soot stream and is therefore costly and time consuming. In addition, after the soot is deposited, the preform must be sintered in a controlled inert atmosphere such as helium. To provide this special equipment for heating in an inert atmosphere is also costly. Unlike the present invention, the soot deposition technique provides a preform with a hole through the center of the preform when the bait rod is removed. This could cause problems in fiber drawing. Still another disadvantage of this technique is disposing of the impurities introduced into the fiber core glass by the flame combustion products during soot deposition. These impurities also include water of hydration which must be removed using a chlorine gas drying process. All of these additional requirements introduce more extensive process control that increase the production cost.
Another fabrication process is known as the modified chemical vapor deposition technique (M.C.V.D.). In this technique, glass precusor vapors are directed through a hollow glass cylinder which is heated sufficiently to start a homogeneous reaction within the glass cylinder. During this reaction. glass particulate matter is formed, deposited on the inside of the glass cylinder, and subsequently fused onto the cylinder and into a glass material by traversing the heat source. Since the starting glass is the outside layer, it may be composed of a material suitable for cladding. This technique, developed at Bell Laboratories, also has some problems related to inefficient deposition rates and starting tube needs which are in turn related to manufacturing economics.
Still another technique for the fabrication of fibers is called a vapor axial deposition process, or more commonly (V.A.D.). This process, described in U.S. Pat. No. 4,062,665, involves simultaneous flame deposition of both core and cladding soots onto the end (axially) of a rotating fused-silica bait rod. As the porous soot preform grows it is slowly drawn through a graphite resistance furnance (carbon heater) where it is consolidated into a transparent glass preform by zone-sintering. This process has all the disadvantages and problems involved with a hydrolysis burner containing dopant halides as described above in the laterial soot deposition technique except in this case there are two (2) hydrolysis burners to control. One burner is for the cladding material and another is for the core material. The process control of the finished preform and the control of both burners must be precise. The average soot collection efficiency for the outside deposition process as described above is approximately 50%. Another problem involves the high hydroxyl impurity content in the fiber core glass introduced by the flame combination products during soot deposition.
What is needed is a method for producing a single mode fiber preform that avoids the problems associated with the prior art. While all of these above techniques can be used to produce a single mode fiber preform, they cannot produce the preform in the same volume and in the same time as in the present invention. In addition, the capital outlay of equipment of the present invention is less than the capital outlay of any one of the previously discussed methods of making fiber optic preforms. This lower capital outlay yeilds a less expensive end product. It is estimated that the present invention will fabricate single mode fibers five (5) times faster than any of the previously discussed techniques at one-half of the cost. The present invention is, therefore, a very cost effective method of fabricating single mode fiber preforms.